The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer circuit structures is known in the art. Such technology is of increasing interest in the production of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Of the conductive metals, gold and copper are of principal interest for the formation of conductive patterns from inks. Copper, of course, is more attractive because it has about the same conductivity as gold but is considerably cheaper. However, there are some problems with the fabrication of conductive lines from copper-based inks. Typically, an ink is comprised of finely divided copper having a particle size of from about 1 to 5 micrometers, a suitable glass frit, also reduced to a fine particle size, and an organic vehicle.
One of the major problems encountered in the fabrication of copper components concerns the removal of the organic vehicle prior to fusing the glass frit at high temperature. Incomplete removal of the organic vehicle will result in organic material being trapped in the fused glass frit. This trapped material, which can decompose into carbonaceous material, can cause changes in the electrical properties of the copper conductor thick film such that a device containing it will be subject to shorts, nonuniform conductivity and the like. Refiring of the copper-containing patterned layers can also decompose remaining organic material into gases which produce bubbles or voids in the final structure. These can likewise cause shorts and are particularly disadvantageous in multilayer structures as they can cause separation of the layers. Further, the presence of bubbles or voids, which can lead to low resistance paths, is disadvantageous in dielectric layers where it is necessary to maintain high resistance.
Were it not for the fact that copper is a readily oxidizable material, all of the organic material could be removed, for example, by simply heating the copper layer to 400.degree.-500.degree. C. in an oxidizing atmosphere. The copper, however, oxidizes at the same time and the degree of oxidation makes it extremely difficult to maintain the quality of the final product. The formation of copper oxide adversely affects the conductivity of the copper structure, the solderability of the fired thick film components and the sintering of the copper particles during the final firing step.
The inevitable presence of some copper oxide on the surface of a copper conductor adversely affects the integrity of an overlying insulator layer. As an overlying insulator layer is fused at high temperature in an inert atmosphere, copper oxide on the surface of the underlying copper conductor will decompose to reform copper and evolve oxygen. This process will, of course, continue as more layers and firings are added to the structure. The evolved oxygen causes loss of integrity of the overlying dielectric layer which is manifest as blistering and peeling of subsequently applied layers and porosity in the dielectric layers which can lead to a reduction in resistance by facilitating copper migration into the dielectric. All of this has a significant adverse effect on the performance of the structure as high dielectric integrity is necessary for good device performance.
In order to reduce the effects caused by firing in an oxidizing atmosphere, it has been suggested to fire thick film inks, particularly copper conductor inks, in nonoxidizing atmospheres, such as nitrogen, to limit the oxidation of the other materials in the ink. The use of a nonoxidizing atmosphere, in addition to being more expensive than oxidizing atmospheres, is not completely satisfactory in that considerable amounts of the organic materials are thermally decomposed to carbonaceous products which remain in the dried layer and can cause shorts and the like, as discussed above.
In Prabhu et al., U.S. Pat. No. 4,619,836, issued Oct. 28, 1986, it is disclosed that the organic vehicle can be removed from a thick film ink prior to firing by treating the dried ink with a suitable plasma, i.e., an oxidizing or reducing plasma. Copper inks are specifically mentioned. It has now been found that, although effective, this method may produce an intolerably high degree of oxidation of the copper conductor in the copper layer if carried out for a time sufficient to assure complete removal of the organic materials therefrom.
In accordance with this invention, a method has been found to plasma treat layers of thick film inks which effectively removes organic materials therefrom without any of above-named disadvantages.